1. Field of the Invention
This invention relates to a methods and structures for elevating a platform above a substrate and for producing a controlled motion of that platform. It also relates to MEMS deformable mirror (“DM”) arrays, and more particularly to long-stroke MEMS deformable mirror arrays for adaptive optics applications.
2. Description of the Related Art
Adaptive optics (“AO”) refers to optical systems that adapt to compensate for disadvantageous optical effects introduced by a medium between an object and an image formed of that object. Horace W. Babcock proposed the concept of adaptive optics in 1953, in the context of mirrors capable of being selectively deformed to correct an aberrated wavefront. See John W. Hardy, Adaptive optics for astronomical telescopes, Oxford series in optical and imaging sciences 16, Oxford University Press, New York, 1998. Since then, deformable mirrors (DM) have been proposed for a variety of AO applications, although they have yet to be implemented in many such proposed applications.
The general operation of a DM is shown schematically in FIG. 1, in which a DM 100 reflects an aberrated wavefront 105, resulting in a desired planar wavefront 110. The DM shape is dynamically adapted to correct the path-length variations of the inbound aberrated wavefront. That is, by selectively deforming the mirror to decrease or increase the path length for specific portions of the aberrated wavefront, the aberrations in the reflected wavefront are corrected. The amount of local displacement needed of the DM surface is generally approximately equal to half the path-length variations in the aberrated wavefront. The exact scale factor depends on the angle at which the aberrated wavefront strikes the deformable mirror.
A prior art AO system is shown schematically in FIG. 2. This example is particularly related to an astronomical telescope application, but the general principles of AO shown here are illustrative of other applications. In FIG. 2, an aberrated wavefront 105 enters the optical system 205 where it is modified as it reflects off a DM 100. Aberrations in the wavefront reflected from the DM are the error signal for a computer-controlled feedback loop. The reflected wavefront 110 enters a dichroic beam splitter 220; the infrared wavelengths pass to a science camera 225 and the visible wavelengths reflect toward a wavefront sensor 230. The wavefront sensor measures the wavefront slope at discrete points and sends these data to a wavefront reconstructor 235. The wavefront reconstructor 235 determines the remaining wavefront aberrations in the corrected wavefront. An actuator control block 240 calculates actuator drive signals to correct the remaining wavefront errors, which are sent from the block 240 to the DM 100, thus closing the feedback loop. In this way, the DM is continuously driven in such a way as to minimize the aberrations in the reflected wavefront, thereby improving image resolution at the science camera.
AO systems have been proposed and demonstrated for improving resolution in a number of imaging applications. In astronomy, for example, AO has been used to correct aberrations introduced by motion of the atmosphere, allowing ground-based telescopes to exceed the resolution provided by the Hubble Space Telescope under some observing conditions. In the field of vision science, AO has been shown to offer benefits, for example, for in-vivo retinal imaging in humans. Here, AO systems can compensate for the aberrations introduced by the eye, improving lateral image resolution by a factor of three and axial resolution by a factor of ten in confocal imagers. This has allowed individual cells to be resolved in living retinal tissue, a capability that was not present before the advent of AO.
In addition to improving image resolution, AO systems can be used to improve confinement of a projected optical beam traveling through an aberrating medium. Examples of applications in this category are free-space optical communication, optical data storage and retrieval, scanning retinal display, and laser-based retinal surgery.
A number of characteristics are commonly used to compare performance of DM designs. Fill-factor is the fraction of the DM aperture that is actively used to correct wavefront aberrations. Mirror stroke is the amount of out-of-plane deformation that can be induced in the DM surface. The number of degrees-of-freedom is a measure of the spatial complexity of the surface shapes the DM is capable of assuming and is related to the number of individual actuators that are used to deform the mirror surface. DM aperture diameter, DM device size, control resolution, operating temperature range, power consumption, frequency response and price are also generally considered when selecting a DM for a given application. For example, astronomical imaging typically requires mirror stroke in the range of a few micrometers, frequency responses in the kilohertz range and aperture sizes on the order of a few centimeters to a few meters. Systems for imaging structures in the human eye, by contrast, generally require mirror stroke on the order of 10 micrometers or greater, frequency responses in the tens to hundreds of Hertz range, and aperture sizes on the order of one centimeter or less.
Despite the advantages outlined above, AO has not been universally adopted, even in the aforementioned applications. Two important factors that have impeded the widespread adoption of AO are the high cost and limited stroke of available DMs.
DM designs can be broadly divided into two classes; continuous-face-sheet designs and segmented designs. Continuous-face-sheet DMs have a reflective surface that is continuous over their whole aperture. The surface is deformed using actuators, typically mounted behind it, that push or pull on it to achieve a desired deformation. This type of DM has been implemented, for example, by mounting an array of piezoelectric actuators to the rear surface of a somewhat flexible glass or ceramic mirror. Because the optical surface is continuous and rather inelastic, large actuation forces are required to deform the mirror, and the resulting mirror stroke is small, typically less than 5 micrometers. The continuous surface also means that the deformation produced by each actuator is not tightly confined to the area of the mirror directly connected to it, but instead may extend across the whole mirror aperture, making precise control of the overall mirror deformation problematic. Because of the way they are constructed, such DMs are also comparatively large, having apertures on the order of 50 mm or greater. This large size precludes their deployment in many optical systems that might otherwise benefit from AO. Their fabrication methods also make these DMs expensive to manufacture and do not permit easy integration of control electronics into the DM structure.
A number of continuous-face-sheet DMs using microfabrication techniques that offer the potential to reduce DM size and cost have been created. Vdovin and Sarro, in “Flexible mirror micromahined in silicon”, Applied Optics, vol. 34, no. 16 (1995), disclose a DM fabricated by assembling a metalcoated silicon nitride membrane above an array of electrodes that are used to deform the membrane by electrostatic attraction.
Bifano et al. disclose an alternative microfabricated continuous-face-sheet DM in “Microelectromechanical Deformable Mirrors”, IEEE Journal of Selected Topics in Quantum Electronics, vol. 5 no. 1 (1999). Their design relies on the removal of a sacrificial layer to create cavities underneath the mirror surface that define the maximum travel range of each mirror actuator. U.S. Pat. No. 6,384,952 to Clark et al. (2002) discloses a continuous-face-sheet DM that employs a mirrored membrane fabricated, for example, from metalcoated silicon nitride and actuated by an array of vertical comb actuators disposed underneath the membrane. Use of vertical comb actuators can provide higher force for a given applied voltage than the parallel plate electrostatic actuators used in other continuous-face-sheet designs.
In contrast to the continuous-face-sheet designs discussed above, segmented DM designs divide the DM aperture into a number of generally planar mirror segments, the angle and height of each segment being controlled by a number of actuators. Segmented designs are advantageous in that they allow the area of influence of each actuator to be tightly confined, simplifying the problem of driving the mirror to a particular desired deformation. Segmenting the mirror surface also eliminates the need to deform a comparatively inflexible optical reflector to produce a desired DM surface shape. Rather, the individual mirror segments are tilted, raised and lowered to form a piecewise approximation of whatever deformation is required to correct the aberrations of the incoming wavefront. Segmenting the surface can therefore result in a lower force requirement for a given surface deformation, enabling the high-stroke DMs that are needed for many AO applications.
A number of inventors have disclosed segmented DM designs that may be constructed using microfabrication techniques. U.S. Pat. No. 6,175,443 to Aksyuk et al. (2001) discloses an array of conductive mirror elements, connected together by linking members that act as supports, suspending the mirror array above an actuating electrode. These linking members also serve to keep the mirror array in an approximately planar configuration when no actuating voltage is applied. Energizing the electrode results in an attractive force between it and the mirror segments, deforming the array into a curved configuration.
U.S. Pat. No. 6,028,689 to Michalicek et al. (2000) discloses an array of mirror segments attached to a substrate by posts, each segment capable of tilting about two axes and also moving vertically, perpendicular to the array, under the influence of applied control voltages.
U.S. Pat. No. 6,545,385 to Miller et al. (2003) discloses methods for elevating a mirror segment above a substrate by supporting it on flexible members that can bend up out of the substrate plane. This provides a large cavity underneath the mirror segment, not limited by the thickness of the sacrificial materials used in its fabrication, and offering the potential for large mirror stroke.
Helmbrecht, in “Micrmirror Arrays for Adaptive Optics”, PhD. Thesis, University of California, Berkeley (2002), discloses a segmented DM for use in AO applications, that exhibits high fill-factor, high mirror quality and offers the potential for high mirror stroke.